1. Field of the Invention
The present invention relates to a method for manufacturing a seamless pipe, and more particularly, to a method for manufacturing a seamless steel pipe and a seamless metal pipe.
2. Description of the Related Arts
In general, a seamless steel pipe is produced by preparing a billet having a round or a square cross-section, forming a hollow shell by a method such as Mannesmann piercing, press piercing or hot extrusion, and rolling the thus-formed hollow shell by a rolling mill such as an elongater, plug mill or a mandrel mill and subjecting the rolled hollow shell to a sizing work performed with a sizer or a stretch reducer, whereby the final pipe product of a predetermined size is obtained (hereinafter called a "continuous rolling process").
The piercing plug for use in Mannesmann piercing and press piercing is always held in contact with a billet heated to a high temperature of 1100 to 1300.degree. C., and the plug is required to sustain a heavy load. Therefore, the piercing plug is damaged during the piercing work. Although a conventional piercing plug is able to withstand several hundreds of piercing cycles when used for piercing a billet made of, for example, a carbon steel (a low-alloy steel), the piercing plug is damaged considerably as shown in FIGS. 4 and 5 when used to pierce billets made of a high-alloy steel exemplified by a stainless steel such as 13 Cr steel, SUS 304 and SUS 316, or a high alloy (hereinafter called a "high-alloy steel or the like"), the main component of which is Cr, Ni or Mo, represented by a high alloy containing, as a main component thereof, not lower than 25 wt % Ni. Thus, the piercing work can be performed only several times. In the worst case, the piercing work cannot be performed. FIGS. 4 and 5 are side views showing a damaged piercing plug. Referring to FIGS. 4 and 5, reference numeral 1 represents a piercing plug itself, 5 represents a deformation of the leading end due to melting, 6 represents a crease damage in the body and 7 represents a seizure of the material of the piercing plug. As described above, the life of the piercing plug, which can be used in hundreds of cycles when used to pierce a carbon steel billet, is extremely shortened if it is used to pierce a billet made of the foregoing high-alloy steel. Therefore, the foregoing problem leads to enlargement of the tool cost, deterioration in the efficiency in the rolling work due to change in the damaged tool, enlargement of the manufacturing cost and, even impossibility of the manufacturing operation.
To overcome the deterioration in the durability of the plug when used to pierce a billet made of the foregoing high-alloy steel, a multiplicity of techniques (1) to (4) below have been suggested:
(1) To use, as the material of the piercing plug, a material, such as Mo (molybdenum), having a greater high-temperature strength than that of the alloy steel to prevent the plug from being damaged due to deformation.
(2) A lubricant is supplied through a portion of the surface of the plug so as to be applied between the hollow shell and the plug to prevent damage taking place due to seizure.
(3) A hard material or the like is allowed to adhere to the surface of the plug by surface treatment to prevent the seizure and abrasion to improve the durability of the plug.
(4) To use a scale produced due to oxidation as a lubricant, the environment in which the heat treatment for the plug and the like are changed to thicken and raise the density of the scale to improve the durability.
The following partial modifications of the foregoing techniques (1) to (4) have been disclosed:
As a modification of art (1), Japanese Patent Laid-Open No. 2-133106 (hereinafter called "prior art 1");
As a modification of art (2), Japanese Patent Laid-Open No. 2-284708 (hereinafter called "prior art 2");
As a modification of art (3), Japanese Patent Laid-Open No. 63-192504 (hereinafter called "prior art 3"); and
As a modification of art (4), Japanese Patent Publication No. 63-54066 (hereinafter called "prior art 4").
It is to be understood, however, that only few cases have been successfully carried out in the prior arts stated above.
The prior art 2 encounters with difficulties in the supply of lubricant in successive piercing cycles, although it can eliminate a seizure between the billet and the plug in at least the first piercing cycle. Supply of lubricant is possible by a different method: namely, through the head of the plug via a passage formed in a plug bar in support of the plug. This method, however, involves a problem in regard to damaging of the plug end or clogging of the same and, hence, cannot be continuously used in actual piercing mills.
A technique considered to be most widely used and in which a piercing plug made of a low-alloy steel, such as 3Cr-1Ni (hereinafter called a "known component") is subjected to heat treatment prior to performing the piercing work to use the produced surface scale as the lubricant is advantageous in view of improving durability similarly to the conventional piercing method. However, the obtained piercing plug can be used in only about 10 cycles which is unsatisfactory as compared with the case where the same is used to pierce common steels. Thus, reduction in the tool cost and improvement in the efficiency in the rolling work cannot be realized.
The prior art 1 relying upon the use of Mo as the plug material has many advantages such as prevention of deformation of the plug itself, prevention of seizure, and so forth. Mo, however, is expensive and the plug made of this material is rather fragile in a temperature range of about 400.degree. C. or less. Due to the foregoing fact and a fact that the plug of the foregoing type can easily be broken due to thermal stress, there are many problems in using industrially.
Although the prior art 3 is able to prevent damage of the plug because a abrasion resistance layer is provided, the abrasion resistance layer made of hard material can easily be cracked due to repeated thermal stress and the layer subjected to the heat treatment separates easily. This method therefore has not yet been matured to such a level as to be practically used on actual machines.
Accordingly, plugs have been disclosed in Japanese Patent Laid-Open No. 62-207503 (hereinafter called a "prior art 5") and Japanese Patent Laid-Open No. 62-244505 (hereinafter called a "prior art 6") as a means capable of elongating the life of the piercing plug and enabling the piercing plug to be manufactured at low cost in which Mo or a Mo alloy or ceramics exhibiting excellent wear resistance is disposed at the leading end of the plug and the body is subjected to the conventional oxidation process or structure.
The plug disclosed in the known arts 5 and 6 (hereinafter called a "composite plug") has the structure such that only the leading portion of the plug, which is applied with a large stress and load, is made of the foregoing strong material, such as Mo, because the cost is increased excessively if the overall body of the piercing plug is made of the foregoing material, and the other portion is made of a low-cost alloy steel to reduce the cost of the tool. The cost per one piercing operation can be reduced to the cost required for the conventional technique and an effect can be improved because the efficiency in the rolling operation can be improved.
However, the foregoing technique cannot reduce the thermal stress taking place due to the friction between the material of the billet and the body of the plug during the operation. Therefore, cracks taking place due to the thermal stress cannot be prevented. Since the oxidation scale applied to the body of the plug is similar to that applied in the conventional techniques, damage of the body due to the piercing work results in the quality and accuracy of a product being excessively deteriorated in its internal surface even if the head of the plug is sound for use. As a result, the manufacturing cost cannot be reduced.
The foregoing problems experienced with the prior arts are as follows:
In the case where the piercing plug is made of the alloy steel of the conventional component or the low-alloy steel, heat applied to the plug from the material to be pierced or rise in the temperature due to heat generated during the work of the material results in deterioration of the strength. Thus, the piercing plug is melted and deformed due to the load applied during the piercing work.
In the case where the piercing plug made of the alloy steel is used to pierce a work piece (billet) which damages the piercing plug, use of the wear-resisting plug having a hardened surface or a plug made of a heat-resisting alloy or Mo or a Mo alloy results mainly in the surface being cracked. Thus, the durability of the plug cannot be improved. In the case where the composite plug is used, damage of the body of the plug inhibits solving of the problem. Therefore, a method of elongating the life of the piercing plug having effects in reducing the manufacturing cost and improvement in the efficiency in the rolling work has not been established yet.
As described above, when a seamless pipe is manufactured from the billet made of the high-alloy steel or the like by the continuous rolling process using Mannesmann piercing, a technique has been desired with which the durability of the piercing plug can be elongated considerably as compared with the conventional method and the manufacturing cost can be reduced.